A. Field of the Invention
This invention relates to systems for maintaining the flow of fluid through flow lines when line pressure is less than or greater than a predetermined maximum or minimum pressure, and more particularly, to a self-contained safety system that does not require an external source of fluid pressure.
B. Description of the Prior Art
In the past, various systems have been developed to control the flow of fluid through a flow line when the line pressure is less than or greater than a predetermined maximum or minimum. One such system, which is described in U.S. Pat. No. 3,043,331, includes a control system that supplies pressurized fluid to an actuator that holds a valve inserted in the flow line in a normally open position. A pair of pilot valves is connected in the system and is adapted to sense line pressure and vent the control system when the line pressure is greater than or less than the maximum or minimum. The venting of the control system causes the actuator to allow the safety valve to close.
All of the prior systems have required an independent external source of control pressure. The need for external control pressure is a shortcoming in certain situations, as, for example, in remote locations where power is unavailable. A further shortcoming of the prior systems is the fact that the control pressure is independent of line pressure. The control pressure must produce a force in the actuator that at least balances the force produced by the line pressure upon the effective area of the safety valve. In the prior systems, the control pressure has been selected to remain constant at a level at least as great as that necessary for the maximum expected line pressure. The control pressure is therefore greater than is necessary for normal line pressure, and therefore requires more pumping energy than is normally required and subjects the control system to excess pressure.
It is therefore an object of the present invention to provide a self-contained safety system that does not require an independent source of control pressure.
It is a further object of the present invention to provide a safety valve system in which the control pressure is proportional to the line pressure.